The subject matter disclosed herein generally relates to a three level inverter and, more particularly, to controlling voltage at the midpoint of the three level inverter.
Aircraft power systems commonly include a generator to generate AC power and a drive system connected the generator for providing excitation to an electric motor for powering actuators and the like. Such drive systems commonly employ rectifier or converter sections to receive the incoming AC power, rectify it, and supply it to a DC bus and an inverter. The drive also may include an inverter that receives DC power from the rectifier/converter and DC bus and provides excitation signals to a machine or load accordingly.
Active front end (AFE) converters in drives are commonly multilevel configurations and employ a pulse width modulated (PWM) switching techniques to convert input AC power to DC output power and provide the output power to the bus. Furthermore, the inverter may also be a multilevel configuration and employ a pulse width modulation technique to then convert the voltage DC power on the bus to AC output currents to drive the load, e.g., a motor. Such active front end converters are typically coupled with input filters, such as LCL filter circuits connected to each power phase. Since the front end rectifier is a switching circuit, the input filter operates to prevent introduction of unwanted harmonic content into the power source. Likewise the inverters may also include filters on the output to provide filtering and isolate unwanted harmonics and common mode interference.
Three level converters and/or inverters have a DC midpoint terminal in addition to a DC positive and a DC negative terminal. Because of the particular arrangement of the DC midpoint, the DC midpoint node voltage is not typically controlled by a power source in order to provide isolation and minimize filter requirements. Therefore, the DC midpoint node voltage can move relative to ground and be difficult to control with three phase loading, especially for unbalanced loads. This imbalance is preferably minimized in order to maintain output current power quality and limit insulated-gate bipolar transistor (IGBT) and DC capacitor voltage stress.
Methods and system elements have been developed to control the DC midpoint voltage. For example, one method of controlling the DC midpoint voltage of a three-level inverter is to utilize a proportional-integral PI regulator. Specifically, the input to the PI regulator is the error in the DC midpoint voltage. A zero-sequence voltage, proportional to the PI regulator output, is applied on the inverter output to reduce the error in the DC midpoint voltage. This loop gain increases as the output power of the inverter increases. Consequently the system may grow unstable at different operating points and present other difficulties for the inverter. Other ways of controlling the DC midpoint voltage in the inverter include filtering and partial isolation schemes. Other methods include direct control of the DC midpoint to selected voltage potential or sophisticated inverter switching techniques.
Accordingly for at least the above discussed reasons, as well as others, there is a desire to provide improved control methods for a multi-level inverter DC midpoint voltage.